When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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Wednesday, July 29, 2009

Subglacial hydrology, basal lubrification, glacier acceleration

It sounds like something they might do to you at a health spa, doesn’t it? But to students of glaciers, basal lubrication is the key that unlocks a long list of puzzles.

Why do precise measurements of glacier motion often show stick-slip behaviour, that is, hours and hours of near motionlessness punctuated by half-hours of rapid movement? Why do some glaciers surge, that is, accelerate suddenly every few decades, flowing rapidly for a year or two before returning, sometimes suddenly but more often gradually, to normal? Why does the landscape of southern Ontario, which I can see from my window, undulate? Why, in the sediment of the northern Atlantic Ocean, are there occasional layers of sand, interrupting the blanket of ultra-fine-grained mud?

The layers of sand beneath the Atlantic are spaced irregularly, 10,000-15,000 years apart, according to the Principle of Superposition, at depths below the sea floor that correspond to the last ice age. They are thin on the European side, thicker towards the northwest, and thickest of all in the neighbourhood of Hudson Strait, which separates Quebec from Baffin Island. The simplest explanation of this pattern is that every so often the bed of the Laurentide Ice Sheet, that covered most of Canada, became much more slippery. Much of its interior was drained by the Hudson Strait Ice Stream, which accelerated occasionally and discharged icebergs in huge numbers. With the icebergs came the sand. All of the plausible accounts of this instability have variations in basal meltwater supply, or possibly just its behaviour, as a critical ingredient.

Around where I live, we are rather proud of our drumlin field. Somebody counted these egg-shaped hills and got up to about four thousand. But geomorphologists now reckon that the tunnel channels are even more interesting. Tunnel channels are drainage networks shaped by subglacial meltwater at the end of the last ice age, after the ice had shaped the drumlins and indeed not long before the ice disappeared altogether. For a long time I simply could not see these things, and I still suspect that the geomorphologists are asking for more meltwater than is probable, but recent evidence from beneath the modern ice sheets is vindicating their interpretations. Now I can see the ancient tunnel valleys in the light of modern ones, apparently hard at work, beneath the Antarctic Ice Sheet.

I don’t know why most glaciers do not surge but a few do. Nor does anyone else. Surging is a phenomenon that has eluded explanation over several decades of concentrated observation and analysis. But we are all positive that subglacial hydrology contains the answer if we can only put together the pieces of the puzzle. The most recent instance of a surging glacier, detected by the U.S. Geological Survey on 3 July 2009, happens also to be a famous glacier -- Malaspina Glacier in Alaska.

Many glaciers go faster in summer, suggesting that meltwater supply has something to do with glacier speed. Where the ice is observed to move in short bursts, there is usually also a suggestion, from one line of evidence or another, that it spends most of the time frozen -- that is, stuck -- to its bed. Slip happens when that immobile state is disturbed, in other words when the bed is lubricated upon the arrival of meltwater. But where does the meltwater come from? And go to?

It might not go anywhere, if the stuff that is moving around is not water but heat. That is, stick-slip may be telling us not about patterns of meltwater flow but about patterns of thawing and freezing. In fact, there may not be any heat moving around either. The melting temperature depends, slightly but measurably, on the confining pressure. So the thaw-freeze patterns could actually be patterns of subtle fluctuations of pressure, not just squeezing the water from one place to another but determining which of the two states, solid or liquid, it is stable in.

It is all very complicated, at scales from sticky patches up to the width of the north Atlantic and beyond. Great fun for glaciologists, but not without consequences for society -- for example, if the Antarctic or Greenland Ice Sheet should decide to do what, according to the lesson from the sand under the Atlantic, the Laurentide Ice Sheet did repeatedly.

"Upstream of the lakes, it flows at two to three metres per year; after passing them, at about 50 metres per year.

"Whether there is a link to climate change is another question. The lakes lie in the eastern portion of Antarctica, where evidence suggests the icecap may be gaining mass rather than losing it...."As this research team puts it: 'The Recovery sub-glacial lakes and the associated Recovery ice stream tributaries have the potential greatly to affect the drainage of the East Antarctic ice sheet, and its influence on sea level rise in the near future.'"

--------Couple questions: at the bottom of the icecap (everywhere, I think) there's enough ice thickness that it's grounded. How close are any areas of the ice to neutral? I realise the water pressure at the bottom of the ice is ---- whatever it is, at a mile or two below sea level.

[There are bits of W Antarctica that are fairly close to neutral - part of its possible instability -W]

Does water under pressure carry more silt than water at 1 atmosphere pressure?I ask because the rapid drumlin article says, yes, they looked through the ice and saw one form, really fast -- these had been thought to be slow creatures.

But -- given that liquid water is flowing along the interface between ice and ground, whatever that ground is (presumably rock) --- how much of what kind of rock flour can that stream carry?

I know it's possible to "fill up" a moving stream's capacity to carry a load -- any time the flow becomes turbulent it drops some and then when it gets laminar it can and will pick up more again. It's one of the conundrums of restoration: if I take a nasty eroding stretch of stream and methodically make check dams and secure eroding banks and plant willow, and otherwise do everything I can to make that stretch of streambed turbulate the flow and be dropping rather than carrying all the sediment it can.

Anytime you turbulate a flow, whatever's flowing drops some of what it's carrying.

If there's a dead air spot on the interface, anyplace a vortex or ripple consistently leaves undisturbed, whatever silt (for a stream), leaves and dust and seeds (for a breeze), or household lint (for a fan).

So --- we're at the bottom of a glacial ice cap. There's a lot of melting way above but we're two miles down and it's been dark and quiet for a while. But every now and then the ice does flow far enough to cause the contact plane to shift downstream a bit.

There will be some flow, where there's excess heat or friction or impurities in the water if anything can change its melting point in those conditions.

We get flows of water; some of them are carrying silt.

That passes through a space where there's a bit of a void, the stream spreads out and slows down and drops what it's carrying.

So, finally, a question -- isn't a drumlin seen happening so fast, likely to be built up by silt filling a void that's melted a bit, on the bottom of the ice, and so going to get silted up as fast as the flowing water going by can provide the silt?

How else could they be happening, under the ice and so fast? And doesn't this lead to some ideas about streamflow rate?

And, has anyone had a look at the Channeled Scablands recently? They were an icecap letting go --- are we sure the water was on top of or behind that ice, or could it have been building up underneath the ice like this?

Because there's one other thing a very silty fast strong flow will do going downhill --- cut away what's in front of it and just rearrange it if it's so full of silt it can't keep any more suspended. A topside melt lake will be mostly water; an under-ice-cap flow must be quite a bit of silt.

Done handwaving; I'll go catch up on the drumlin stories. Turns out they're seen on Mars, resembling those in the Scablands. Hmmmm.

Does this make sense?"... Martin Siegert, a glaciologist at the University of Bristol in England. "\...

The melting point of ice in environments such as Lake Vostok is related to the thickness of the ice above the water. The melting point is colder under thicker ice, as it is at the northern end of the lake.

The water that melts at the northern end will thus be colder and less dense than water at the southern end. "The density contrast between these waters will cause the circulation," said Siegert.-------I wonder why the melting point would be colder under thicker ice, and if that relationship is linear, or describes only the Lake Vostok depth conditions.(Clipped from a BBC story, lost the cite, sorry).

[Because of the pressure effect. Pressure will melt ice, you know that; ie the melt point gets lower under pressure -W]

" ... the WAIS is considered unstable because a large portion of it floats on water above the sea floor. For this reason, scientists suspect that the WAIS is particularly sensitive to global climate change, and they have long debated whether global warming would cause the WAIS to collapse. ...

I'd speculate the whole idea of meltwater lakes sitting on top of icecaps and spilling over is due for a revision --- and that we'll be looking at things like drumlins completely differently now that we know they can form rapidly under ice. It still has to be happening from deposition by flowing water --- but it's not surface water.

Looking at the radar maps of the ground under the ice, what I see is the major areas below sea level looking like places not pushed down dramatically by the overlying ice, but like river drainage channels. As the ice accumulated and moved, now that we know about under ice flows, the icecap would push whatever rock flour and warm-epoch silt out toward the edges, carrying it along with meltwater.

What do you see looking at the radar maps? Looks to me like --- clear shapes toward the middle; less and less clear out to the edges along each likely river course, and big smooth rounded banks of what I'd expect to be extruded silt/rock flour along the edges.

I think under the ice caps water is flowing 'uphill' from the basins around the center, radially, and as it spreads out it flows slower, so the farther it goes toward the circumference the more silt it drops.

Fill a deep bowl with peanut butter, put a shallower bowl on the top, push down ....

Okay, enough speculation from the uninformed and uneducated moi. Just wondering.

Belatedly, I find New Scientist covered all these ideas (except they haven't quoted anyone anticipating my Channeled Scablands speculation, you read that first here) in their special December 2-8, 2006, special issue: "Hidden World Beneath Antarctica's Ice."

It's really quite good. Other than being quietly overwhelming.

"Water moves in mysterious ways. The weight of the ice squeezing downwards counts for much more than local hills and valleys in telling water where to go. 'You can have lakes sloping down the sides of mountains, you can have uphill waterfalls, it's wacky' [Don Blankenship, geophysicist at U. Texas] ....Blankenship fears that warming since the end of the last ice age has melted the base of the ice, and this may already be priming some parts of the ice sheet to slip. East Antarctica could be ready to open its floodgates.

"David Marchant from Boston University believes this may have happened before. .... one place in particular, a tortured landscape of channels and pits known as the Labyrinth .... sinuous .... often potholes .... channels that stop abruptly .... what you'd expect if they had been made by water that then flowed off down a different path, ... or plunged [that's in an upward direction --hr] into the overlying ice.

"He became convinced that the Labyrinth had been carved by a massive under-ice flood, and he published his ideas in July (Geology, v34, p. 513). .... potholes that are 200 metres across and 50 metres deep,' says Marchant. 'They are just enormous features. They're the largest potholes in the world. The water quickly stripped away all sedimentary rocks, and then lifted blocks of granite bedrock more than 2 metres wide ..."

----Okay, I think this is serious stuff. Anyone found any more about it?

"... By about 1990 the way that glaciologists envisagedbasal water flow had greatly changed, largely owing tothe realization that R channels could not form the basisfor an explanation of observations from surging Varie-gated Glacier [Kamb et al., 1985] and rapidly moving icestream B in Antarctica [Blankenship et al., 1987]. Theo-ries were developed to elucidate the hydraulics of waterflow through linked cavities [Walder, 1986; Kamb, 1987],deformable till [Alley et al., 1987], and till-floored chan-nels [Walder and Fowler, 1994]....

To summarize, the drainage system under any givenglacier comprises several or all of the morphologicallydistinct components described in this section. A slow,nonarborescent drainage system, comprising a mixtureof elements including cavities, permeable till, andconduits incised into the bed (i.e., Nye channels and canals),probably covers most of the glacier bed and is nearlyfixed relative to the bed. The water pressure in the slowdrainage system is commonly close to the ice-overburden pressure.

... early conclusions drawn by geologists at Andrill (Antarctic Geological Drilling), the multinational consortium leading the project, which recently released preliminary data from the drilling on its Web site. ...

A first look at conditions that prevailed five million years ago

"This time we were able to drill into layers representing the period between five and 12 million years ago," Andrill team member and geologist Lothar Viereck-Götte told SPIEGEL ONLINE. What these unique ice cores revealed about temperature changes in the last 5 million years was both surprising and new, says Viereck-Götte, who calls the results "horrifying." The data suggests "the ice caps are substantially more mobile and sensitive than we had assumed."

Re the latest release from the IPCC:---- quote----Change 3: Deleting a threat:The scientists originally included this statement about changes caused by retreating glaciers:enlargement and increased numbers of glacial lakes, with increased risk of outburst floods

The final draft wound up reading:enlargement and increased numbers of glacial lakes---- end quote -----

Subglacial floods (jökulhlaups) are well documented as occurring beneath present day glaciers and ice caps. In addition, it is known that massive floods have occurred from ice-dammed lakes proximal to the Laurentide ice sheet during the last ice age, and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago. We propose that drainage of lakes from beneath ice sheets will generally occur in a time-periodic fashion, and that such floods can be of severe magnitude. Such hydraulic eruptions are likely to have caused severe climatic disturbances in the past, and may well do so in the future.

Lo! It's News! (Well, it's about Greenland, so I'm still just speculating that it's happening in Antarctica too, and happened underneath the continental glaciers elsewhere like in Idaho instead of water pooling on top of them as often pictured.)

Note per last line of the abstract that nobody has thought about this possibility til now :-)

Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA

Abstract

The floating ice shelf in front of Petermann Glacier, in northwest Greenland, experiences massive bottom melting that removes 80% of its ice before calving into the Arctic Ocean. Detailed surveys of the ice shelf reveal the presence of 1-2 km wide, 200-400 m deep, sub-ice shelf channels, aligned with the flow direction and spaced by 5 km. We attribute their formation to the bottom melting of ice from warm ocean waters underneath. Drilling at the center of one of channel, only 8 m above sea level, confirms the presence of ice-shelf melt water in the channel. These deep incisions in ice-shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously. ....

[Interesting, true, but losing 80% is an awful lot. It hardly matters if they break up after that :-) nb this can only apply to ice shelves, not the main ice sheet -W]

[The channels. They are from oceanic heat: "We attribute their formation to the bottom melting of ice from warm ocean waters underneath" -W]

I'd think the channels observed out at the edges of the Greenland ice would likely have counterparts under the Antarctic -- the shape would seem to be from meltwater flowing out rather than melting along the edge from warm seawater.

I recall (maybe left links earlier above or in the old Prometheus thread) that liquid water has been described there from cameras lowered into boreholes to the base of the ice, in surprising large voids, and flows of water observed under the ice sheets would, I'd think, create similar channels.

Robotic Telescope Installed on Antarctica PlateauPosted by Zonk on Wednesday February 06, @03:44PM

Robotics Space Science

Reservoir Hill writes "Antarctica claims some of the best astronomical sky conditions in the world -- devoid of clouds with steady air that makes for clear viewing. The very best conditions unfortunately lie deep in the interior on a high-altitude plateau called Dome A. With an elevation of up to 4,093m, it's known as the most unapproachable point in the earth's southernmost region. Now astronomers in a Chinese scientific expedition have set up an experimental observatory at Dome A after lugging their equipment across Antarctica with the help of Australia and the US. The observatory will hunt for alien planets, while also measuring the observing conditions at the site to see if it is worth trying to build bigger observatories there. The observatory is automated, pointing its telescopes on its own while astronomers monitor its progress from other locations around the world via satellite link. PLATO is powered by a gas generator, and has a 4000-litre tank of jet fuel to keep it running through the winter. The observatory will search for planets around other stars using an array of four 14.5-centimetre telescopes called the Chinese Small Telescope Array (CSTAR). Astronomers hope to return in 2009 with new instruments, including the Antarctica Schmidt Telescopes (AST-3), a trio of telescopes with 0.5-metre mirrors, which will be more sensitive to planets than CSTAR."

... The paired surface temperature and gravity data confirm a strong connection between melting on ice sheet surfaces in areas below 6,500 feet in elevation, and ice loss throughout the ice sheet's giant mass. The result led Hall's team to conclude that the start of surface melting triggers mass loss of ice over large areas of the ice sheet.

The beginning of mass loss is highly sensitive to even minor amounts of surface melt. Hall and her colleagues showed that when less than two percent of the lower reaches of the ice sheet begins to melt at the surface, mass loss of ice can result. For example, in 2004 and 2005, the GRACE satellites recorded the onset of rapid subsurface ice loss less than 15 days after surface melting was captured by the Terra satellite.

The MODIS instrument acquired this image of melt ponds on Greenland's western coast in June, 2006. The MODIS instrument acquired this image of melt ponds on Greenland's western coast in June, 2006. The ponds appear as dark blue dots on the aqua blue background.

"We're seeing a close correspondence between the date that surface melting begins, and the date that mass loss of ice begins beneath the surface," Hall said. "This indicates that the meltwater from the surface must be traveling down to the base of the ice sheet -- through over a mile of ice -- very rapidly, where its presence allows the ice at the base to slide forward, speeding the flow of outlet glaciers that discharge icebergs and water into the surrounding ocean."

-----How will the modelers handle this behavior?

-------I recall this:

[Response: Dynamics are as important as thermodynamics here. Recent evidence (e.g. as reviewed by us a few months back) suggests that the demise of large parts of the major ice sheets could potentially take place far faster-on timescales of perhaps several centuries-due to the influence of ice sheet dynamics. For example, crevices at the surface of the ice sheet are now known to sometimes penetrate all the way down to the bottom of the ice sheet forming channels ("moulins") that allow surface meltwater to reach the bottom of the ice sheet, where it lubricates the ice, allowing it to stream into the ocean at velocities potentially far greater than once envisioned. These processes are still far from perfectly understood, because they require a representation of the fairly complicated rheology involved in ice sheet dynamics. But it appears far more likely that a better understanding of these processes will act to revised our estimates of ice sheet collapse timescales downward, rather than upward. - mike]

... the researchers spent most of their time driving skidoos across the flat, featureless ice.

"We drove skidoos over it for something like 2,500km each and we didn't see a single piece of topography."

Rob Bingham was towing a radar on a 100m-long line and detecting reflections from within the ice using a receiver another 100m behind that.

The signals are revealing ancient flow lines in the ice. The hope is to reconstruct how it moved in the past....Throughout the 1990s, according to satellite measurements, the glacier was accelerating by around 1% a year. Julian Scott's sensational finding this season is that it now seems to have accelerated by 7% in a single season, sending more and more ice into the ocean.

"The measurements from last season seem to show an incredible acceleration, a rate of up to 7%. That is far greater than the accelerations they were getting excited about in the 1990s."

Quoting in full, because it answers a lot of my questions in simple clear language of few syllables:

# Mauri Pelto Says:28 April 2008 at 2:52 PM

Lakes form at the bottom of a glacier or on the surface. Because ice crystals deform under pressure, and pressure is substantial within a glacier or ice sheet it is not possible to have substantial void volumes. Ice under pressure would deform and flow into this void. This happens to much of the seasonal hydrology system each winter. Without water flow to keep tunnels open, they close, then in spring maximum water pressures often occur befor the conduit system redevelops. Once opened the flowing meltwater can maintain these narrow conduits. However, the meltwater does not have enough heat to melt much. At the base of the glaciers even in the summer next to these streams, you will see new ice coating the bedrock in places. The moulin ice riddling is science fiction. No ice sheet or glacier collapses due to riddling by moulins. I still see a persistent misconception about the ability of meltwater to melt glacier ice and riddle the glacier with holes. I work on glaciers with lots of melt and they are not weakened by all the meltwater drainage. The meltwater is not a very capable melter of ice. Ice is unlike rock which does not deform under the pressure and temperatures observed on glaciers.-------------------------------------

in late 2006, the Andrill team discovered undisturbed deposits 15 kilometers (9 miles) outside the research station near the Mount Erebus volcano.

A first look at conditions that prevailed five million years ago

"This time we were able to drill into layers representing the period between five and 12 million years ago," Andrill team member and geologist Lothar Viereck-Götte told SPIEGEL ONLINE.

What these unique ice cores revealed about temperature changes in the last 5 million years was both surprising and new, says Viereck-Götte, who calls the results "horrifying." The data suggests "the ice caps are substantially more mobile and sensitive than we had assumed."

"The idea that the ocean here was ice-free for almost a million years is completely new," says Viereck-Götte. Besides, he adds, the melting that occurred about 5 million years ago can be seen in the context of a prehistoric climate shift.

According to Viereck-Götte, "massive melting" must have occurred in the Antarctic during the so-called Miocene-Pliocene warming. The cause sounds anything but massive. Based on isotope analyses from various locations worldwide, paleoclimatologists know that the average global temperature in the oceans increased by only two to three degrees Celsius (3.6-5.4 degrees Fahrenheit) -- a seemingly minor change. Nevertheless this change in temperature, according to the new Andrill ice core, led to an ice-free Ross Sea.

For researchers the clue lies in tiny microorganisms known as diatoms, which cannot survive in water that is covered by ice. But they were found in the core representing an uninterrupted period of 1 million years.

"We would never have thought that this system is so sensitive," says Viereck-Götte. The consequences of an ice-free Ross Sea would be far-reaching, not just for sea levels.

Abstract: Estimates of sea ice extent based on satellite observations show an increasing Antarctic sea ice cover from 1979 to 2004 even though in situ observations show a prevailing warming trend in both the atmosphere and the ocean. This riddle is explored here using a global multicategory thickness and enthalpy distribution sea ice model coupled to an ocean model. Forced by the NCEP-NCAR reanalysis data, the model simulates an increase of 0.20 x 10(12) m(3) yr(-1) (1.0% yr(-1)) in total Antarctic sea ice volume and 0.084 x 10(12) m(2) yr(-1) (0.6% yr(-1)) in sea ice extent from 1979 to 2004 when the satellite observations show an increase of 0.027 x 10(12) m(2) yr(-1) (0.2% yr(-1)) in sea ice extent during the same period. The model shows that an increase in surface air temperature and downward longwave radiation results in an increase in the upper-ocean temperature and a decrease in sea ice growth, leading to a decrease in salt rejection from ice, in the upper-ocean salinity, and in the upper-ocean density. The reduced salt rejection and upper-ocean density and the enhanced thermohaline stratification tend to suppress convective overturning, leading to a decrease in the upward ocean heat transport and the ocean heat flux available to melt sea ice. The ice melting from ocean heat flux decreases faster than the ice growth does in the weakly stratified Southern Ocean, leading to an increase in the net ice production and hence an increase in ice mass. This mechanism is the main reason why the Antarctic sea ice has increased in spite of warming conditions both above and below during the period 1979-2004 and the extended period 1948-2004.